am I right in thinking all the dedicated ones are for just 35mm film?
Yes. IIRC Nikon used to (maybe still do) make ones that could scan upto 5x4" negs. Minolta used to make some (like my dinosaur) that could accept APS.
Any consumer priced flatbed will be quoting their maximum interpolated resolution. Quite often the real resolution will only be something like 600 x 2400 dpi. The 2400 dpi is from the scanning element. The 600 dpi is from the mechanism that moves the element.
If your dad is a keen photographer and wants to archive his entire collection a dedicated film scanner is the way to go. Of course he would need to invest in some extra storage and backup solution. If all he wants to do is catalogue and print some of his collection then any scanner will do.